Cathode materials for secondary (rechargeable) lithium batteries

ABSTRACT

The invention relates to materials for use as electrodes in an alkali-ion secondary (rechargeable) battery, particularly a lithium-ion battery. The invention provides transition-metal compounds having the ordered-olivine or the rhombohedral NASICON structure and the polyanion (PO 4 ) 3-  as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.

This application claims priority to provisional patent application No. 60/032,346 filed Dec. 4, 1996, and provisional patent application No. 60/016,060, filed Apr. 23, 1996. The entire text of each of the above-referenced disclosures is specifically incorporated by reference herein without disclaimer. The Robert A. Welch Foundation, Houston, Tex., supported research related to the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to secondary (rechargeable) alkali-ion batteries. More specifically, the invention relates to materials for use as electrodes for an alkali-ion battery. The invention provides transition-metal compounds having the ordered olivine or the rhombohedral NASICON structure and containing the polyanion (PO₄)³⁻ as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.

2. Description of the Related Art

Present-day lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li⁺ ions to the Li⁺ -ion electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li⁺ ions are inserted reversibly from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible. On charge, removal of electrons from the cathode by an external field releases Li⁺ ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li⁺ ions back into the anode to restore it to its original composition.

Present-day rechargeable lithium-ion batteries use a coke material into which lithium is inserted reversibly as the anode and a layered or framework transition-metal oxide is used as the cathode host material (Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/or Ni are expensive and may degrade due to the incorporation of unwanted species from the electrolyte. Oxides such as Li₁±x Mn₂ !O₄, which has the M₂ !O₄ spinel framework, provide strong bonding in three dimensions and an interconnected interstitial space for lithium insertion. However, the small size of the O²⁻ ion restricts the free volume available to the Li⁺ ions, which limits the power capability of the electrodes. Although substitution of a larger S²⁻ ion for the O²⁻ ion increases the free volume available to the Li⁺ ions, it also reduces the output voltage of an elementary cell.

A host material that will provide a larger free volume for Li⁺ -ion motion in the interstitial space would allow realization of a higher lithium-ion conductivity σ_(Li), and hence higher power densities. An oxide is needed for output voltage, and hence higher energy density. An inexpensive, non-polluting transition-metal atom would make the battery environmentally benign.

SUMMARY OF THE INVENTION

The present invention meets these goals more adequately than previously known secondary battery cathode materials by providing oxides containing larger tetrahedral oxide polyanions forming 3D framework host structures with octahedral-site transition-metal oxidant cations, such as iron, that are environmentally benign.

The present invention provides electrode material for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte. The cell may additionally include an electrode separator. As used herein, "electrochemical cell" refers not only to the building block, or internal portion, of a battery but is also meant to refer to a battery in general. Although either the cathode or the anode may comprise the material of the invention, the material will preferably be useful in the cathode.

Generally, in one aspect, the invention provides an ordered olivine compound having the general formula LiMPO₄, where M is at least one first row transition-metal cation. The alkali ion Li⁺ may be inserted/extracted reversibly from/to the electrolyte of the battery to/from the interstitial space of the host MPO₄ framework of the ordered-olivine structure as the transition-metal M cation (or combination of cations) is reduced/oxidized by charge-compensating electrons supplied/removed by the external circuit of the battery in, for a cathode material, a discharge/charge cycle. In particular, M will preferably be Mn, Fe, Co, Ti, Ni or a combination thereof. Examples of combinations of the transition-metals for use as the substituent M include, but are not limited to, Fe_(1-x) Mn_(x), and Fe_(1-x) Ti_(x), where 0<x<1.

Preferred formulas for the ordered olivine electrode compounds of the invention include, but are not limited to LiFePO₄, LiMnPO₄, LiCoPO₄, LiNiPO₄, and mixed transition-metal comounds such as Li_(1-2x) Fe_(1-x) Ti_(x) PO₄ or LiFe_(1-x) Mn_(x) PO₄, where 0<x<1. However, it will be understood by one of skill in the art that other compounds having the general formula LiMPO₄ and an ordered olivine structure are included within the scope of the invention.

The electrode materials of the general formula LiMPO₄ described herein typically have an ordered olivine structure having a plurality of planes defined by zigzag chains and linear chains, where the M atoms occupy the zigzag chains of octahedra and the Li atoms occupy the linear chains of alternate planes of octahedral sites.

In another aspect, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials comprise a rhombohedral NASICON material having the formula Y_(x) M₂ (PO₄)₃, where 0<x<5. Preferably, the compounds of the invention will be useful as the cathode of a rechargeable electrochemical cell. The alkali ion Y may be inserted from the electrolyte of the battery to the interstitial space of the rhombohedral M₂ (XO₄)₃ NASICON host framework as the transition-metal M cation (or combination of cations) is reduced by charge-compensating electrons supplied by the external circuit of the battery during discharge with the reverse process occurring during charge of the battery. While it is contemplated that the materials of the invention may consist of either a single rhombohedral phase or two phases, e.g. orthorhombic and monoclinic, the materials are preferably single-phase rhombohedral NASICON compounds. Generally, M will be at least one first-row transition-metal cation and Y will be Li or Na. In preferred compounds, M will be Fe, V, Mn, or Ti and Y will be Li.

Redox energies of the host M cations can be varied by a suitable choice of the XO₄ polyanion, where X is taken from Si, P, As, or S and the structure may contain a combination of such polyanions. Tuning of the redox energies allows optimization of the battery voltage with respect to the electrolyte used in the battery. The invention replaces the oxide ion O²⁻ of conventional cathode materials by a polyanion (XO₄)^(m-) to take advantage of (1) the larger size of the polyanion, which can enlarge the free volume of the host interstitial space available to the alkali ions, and (2) the covalent X--O bonding, which stabilizes the redox energies of the M cations with M--O--X bonding so as to create acceptable open-circuit voltages V_(OC) with environmentally benign Fe³⁺ /Fe²⁺ and/or Ti⁴⁺ /Ti³⁺ or V⁴⁺ /V³⁺ redox couples.

Preferred formulas for the rhombohedral NASICON electrode compounds of the invention include, but are not limited to those having the formula Li_(3+x) Fe₂ (PO₄)₃, Li_(2+x) FeTi(PO₄)₃, Li_(x) TiNb(PO₄)₃, and Li_(1+x) FeNb(PO₄)₃, where 0<x<2. It will be understood by one of skill in the art that Na may be substituted for Li in any of the above compounds to provide cathode materials for a Na ion rechargeable battery. For example, one may employ Na_(3+x) Fe₂ (PO₄)₃, Na_(2+x) FeTi(PO₄)₃, Na_(x) TiNb(PO₄)₃ or Na_(1+x) FeNb(PO₄)₃, where 0<x<2, in a Na ion rechargeable battery. In this aspect, Na⁺ is the working ion and the anode and electrolyte comprise a Na compound.

Compounds of the invention having the rhombohedral NASICON structure form a framework of MO₆ octahedra sharing all of their comers with XO₄ tetrahedra (X═Si, P, As, or S), the XO₄ tetrahedra sharing all of their comers with octahedra. Pairs of MO₆ octahedra have faces bridged by three XO₄ tetrahedra to form "lantern" units aligned parallel to the hexagonal c-axis (the rhomobhedral 111! direction), each of these XO₄ tetrahedra bridging to two different "lantern" units. The Li⁺ or Na⁺ ions occupy the interstitial space within the M₂ (XO₄)₃ framework. Generally, Y_(x) M₂ (XO₄)₃ compounds with the rhombohedral NASICON framework may be prepared by solid-state reaction of stoichiometric proportions of the Y, M, and XO₄ groups for the desired valence of the M cation. Where Y is Li, the compounds may be prepared indirectly from the Na analog by ion exchange of Li⁺ for Na⁺ ions in a molten LiNO₃ bath at 300° C. For example, rhombohedral LiTi₂ (PO₄)₃ may be prepared from intimate mixtures of Li₂ CO₃ or LiOH·H₂ O, TiO₂, and NH₄ H₂ PO₄ ·H₂ O calcined in air at 200° C. to eliminate H₂ O and CO₂ followed by heating in air for 24 hours near 850° C. and a further heating for 24 hours near 950° C. However, preparation of Li₃ Fe₂ (PO₄)₃ by a similar solid-state reaction gives the undesired monoclinic framework. To obtain the rhombohedral form, it is necessary to prepare rhombohedral Na₃ Fe₂ (PO₄)₃ by solid-state reaction of NaCO₃, Fe{CH₂ COOH}₂ and NH₄ H₂ PO₄ ·H₂ O, for example. The rhombohedral form of Li₃ Fe₂ (PO₄)₃ is then obtained at 300° C. by ion exchange of Li⁺ for Na⁺ in a bath of molten LiNO₃. It will be understood by one of skill in the art that the rhombohedral Na compounds will be useful as cathode materials in rechargeable Na ion batteries.

In another aspect of the invention, the rhombohedral NASICON electrode compounds may have the general formula Y_(x) M₂ (PO₄)_(y) (XO₄)_(3-y), where 0<y≦3, M is a transition-metal atom, Y is Li or Na, and X═Si, As, or S and acts as a counter cation in the rhombohedral NASICON framework structure. In this aspect, the compound comprises a phosphate anion as at least part of an electrode material. In preferred embodiments, the compounds are used in the cathode of a rechargeable battery. Preferred compounds having this general formula include, but are not limited to Li_(1+x) Fe₂ (SO₄)₂ (PO₄), where 0≦x≦1.

The rhombohedral NASICON compounds described above may typically be prepared by preparing an aqueous solution comprising a lithium compound, an iron compound, a phosphate compound and a sulfate compound, evaporating the solution to obtain dry material and heating the dry material to about 500° C. Preferably, the aqueous starting solution comprises FeCl₃, (NH₄)₂ SO₄, and LiH₂ PO₄.

In a further embodiment, the invention provides electrode materials for a rechargeable electrochemical cell comprising an anode, a cathode and an electrolyte, with or without an electrode separator, where the electrode materials have a rhombohedral NASICON structure with the general formula A_(3-x) V₂ (PO₄)₃. In these compounds, A may be Li, Na or a combination thereof and 0≦x≦2. In preferred embodiments, the compounds are a single-phase rhombohedral NASICON material. Preferred formulas for the rhombohedral NASICON electrode compounds having the general formula A_(3-x) V₂ (PO₄)₃ include, but are not limited to those having the formula Li_(2-x) NaV₂ (PO₄)₃, where 0≦x≦2.

The rhombohedral NASICON materials of the general formula A_(3-x) V₂ (PO₄)₃ may generally be prepared by the process outlined in FIG. 9. Alternatively, Li₂ NaV₂ (PO₄)₃ may be prepared by a direct solid-state reaction from LiCO₃, NaCO₃, NH₄ H₂ PO₄ ·H₂ O and V₂ O₃.

In a further aspect, the invention provides a secondary (rechargeable) battery where an electrochemical cell comprises two electrodes and an electrolyte, with or without an electrode separator. The electrodes are generally referred to as the anode and the cathode. The secondary batteries of the invention generally comprise as electrode material, and preferably as cathode material, the compounds described above. More particularly, the batteries of the invention have a cathode comprising the ordered olivine compounds described above or the rhombohedral NASICON compounds described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and are included to demonstrate further certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.

FIG. 1. FIG. 1 shows a typical polarization curve for the battery voltage V vs. the I delivered across a load. The voltage drop (V_(OC) -V).tbd.η(I) of a typical curve is a measure of the battery resistance R_(b) (I). The in-terfacial voltage drops saturate in region (i). The slope of the curve in region (ii) is dV/dI≈R_(e1) +R_(C) (A)+R_(C) (C), the sums of the electrolyte resistance R_(e1) and the current-collector resistances at the anode and cathode. Region (iii) is diffusion-limited. At the higher currents I, normal processes do not bring ions to or remove them from the electrode/electrolyte interfaces rapidly enough to sustain an equilibrium reaction.

FIG. 2A, 2B and 2C. FIG. 2A shows discharge/charge curves at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1-x) FePO₄ as cathode and lithium as anode. A plateau at 3.4V corresponds to the Fe³⁺ /Fe²⁺ redox couple relative to the lithium anode. A plateau at 4.1 V corresponds to the Mn³⁺ /Mn²⁺ couple. FIG. 2B shows discharge/charge curves at 0.05 mA·cm⁻² (1.13 mA·g⁻¹) for the olivine Li_(1-x) Fe₀.5 Mn₀.5 PO₄ as cathode relative to a lithium anode. FIG. 2C shows discharge/charge curves vs. lithium at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(x) FePO₄.

FIG. 3. FIG. 3 shows discharge/charge curves of an FePO₄ /LiClO₄ +PC+DME/Li coin cell at 185 mA·g⁻¹ for FePO₄ prepared by chemical extraction of Li (delithiation) from LiFePO₄.

FIG. 4. Schematic representation of the motion of LiFePO₄ /FePO₄ interface on lithium insertion in to a particle of FePO₄.

FIG. 5A and 5B. FIG. 5A shows the rhombohedral R3c (NASICON) framework structure of Li₃ Fe₂ (PO₄)₃ prepared by ion exchange from Na₃ Fe₂ (PO₄)₃ ; FIG. 5B shows the monoclinic P2₁ /n framework structure of Li₃ Fe₂ (PO₄)₃ prepared by solid-state reaction.. The large, open three-dimensional framework of FeO₆ octahedra and PO₄ tetrahedra allows an easy diffusion of the lithium ions.

FIG. 6A and 6B. FIG. 6A shows discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for rhombohedral Li_(3+x) Fe₂ (PO₄)₃ where 0<x<2. The shape of the curve for lithium insertion into rhombohedral Li_(3+x) Fe₂ (PO₄)₃ is surprisingly different from that for the monoclinic form. However, the average V_(OC) at 2.8 V remains the same. The Li⁺ -ion distribution in the interstitial space appears to vary continuously with x with a high degree of disorder. FIG. 6B shows discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for monoclinic Li_(3+x) Fe₂ (PO₄)₃ where 0<x<2.

FIG. 7A and 7B. FIG. 7A shows discharge curves vs. a lithium anode at current densities of 0.05-0.5 mA·cm⁻² for rhombohedral Li_(3+x) Fe₂ (PO₄)₃. A reversible capacity loss on increasing the current density from 0.05 to 0.5 mA·cm⁻² is shown. This loss is much reduced compared to what is encountered with the monoclinic system. FIG. 7B shows discharge curves at current densities of 0.05-0.5 mA·cm⁻² for monoclinic Li_(3+x) Fe₂ (PO₄)₃.

FIG. 8. FIG. 8 shows discharge/charge curves at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the rhombohedral Li_(x) NaV₂ (PO₄)₃. Rhombohedral Li₂ NaV₂ (PO₄)₃ reversibly intercalates 1.5 Li per formula unit for a discharge capacity of 100 mAh·g⁻¹ with average closed-circuit voltage of 3.8 V vs. a lithium anode.

FIG. 9. FIG. 9 illustrates the solid-state synthesis of Li₂ NaV₂ (PO₄)₃ having the rhombohedral NASICON framework.

FIG. 10 FIG. 10 shows discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for rhombohedral Li_(1+x) Fe₂ (PO₄)(SO₄)₂ where 0≦x≦2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Present-day secondary (rechargeable) lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. It is important that the chemical reactions at the anode and cathode of a lithium secondary battery be reversible. On discharge, the metallic anode supplies Li⁺ ions to the Li⁺ -ion electrolyte and electrons to the external circuit. The cathode is a host compound into/from which the working Li⁺ ion of the electrolyte can be inserted/extracted reversibly as a guest species over a large solid-solubility range (Goodenough 1994). When the Li⁺ ions are inserted as a guest species is into the cathode, they are charge-compensated by electrons from the external circuit. On charge, the removal of electrons from the cathode by an external field releases Li⁺ ions back to the electrolyte to restore the parent host structure. The resultant addition of electrons to the anode by the external field attracts charge-compensating Li⁺ ions back into the anode to restore it to its original composition.

The present invention provides new materials for use as cathodes in lithium secondary (rechargeable) batteries. It will be understood that the anode for use with the cathode material of the invention may be any lithium anode material, such as a reductant host for lithium or elemental lithium itself. Preferably, the anode material will be a reductant host for lithium. Where both the anode and cathode are hosts for the reversible insertion or removal of the working ion into/from the electrolyte, the electrochemical cell is commonly called a "rocking-chair" cell. An implicit additional requirement of a secondary battery is maintenance not only of the electrode/electrolyte interfaces, but also of electrical contact between host particles, throughout repeated discharge/recharge cycles.

Since the volumes of the electrode particles change as a result of the transfer of atoms from one to another electrode in a reaction, this requirement normally excludes the use of a crystalline or glassy electrolyte with a solid electrode. A non-aqueous liquid or polymer electrolyte having a large energy-gap window between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) is used with secondary lithium batteries in order to realize higher voltages. For example, practical quantities of very ionic lithium salts such as LiClO₄, LiPF₄ and LiPF₆ can be dissolved in empirically optimized mixtures of propylene carbonate (PC), ethylene carbonate (EC), or dimethyl carbonate (DMC) to provide acceptable electrolytes for use with the cathodes of the invention. It will be recognized by those of skill in the art that the (ClO₄)⁻ anion is explosive and not typically suitable for commercial applications.

General Design Considerations

The power output P of a battery is the product of the electric current I delivered by the battery and the voltage Vacross the negative and positive posts (equation 1).

    P=IV                                                       (1)

The voltage V is reduced from its open-circuit value V_(OC) (I=0) by the voltage drop IR_(b) due to the internal resistance R_(b) of the battery (equation 2).

    V=V.sub.OC -IR.sub.b                                       (2)

The open-circuit value of the voltage is governed by equation 3.

    V.sub.OC =(μ.sub.A -μ.sub.C)/(-nF)<5V                (3)

In equation 3, n is the number of electronic charges carried by the working ion and F is Faraday's constant. The magnitude of the open-circuit voltage is constrained to V_(OC) <5V not only by the attainable difference μ_(A) -μ_(C) of the electrochemical potentials of the anode reductant and the cathode oxidant, but also by the energy gap E_(g) between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of a liquid electrolyte or by the energy gap E_(g) between the top of the valence band and the bottom of the conduction band of a solid electrolyte.

The chemical potential μ_(A), which is the Fermi energy ε_(F) of a metallic-reductant anode or the HOMO of a gaseous or liquid reductant, must lie below the LUMO of a liquid electrolyte or the conduction band of a solid electrolyte to achieve thermodynamic stability against reduction of the electrolyte by the reductant. Similarly, the chemical potential μ_(C), which is the LUMO of a gaseous or liquid oxidant or the Fermi energy of a metallic-oxidant cathode, must lie above the HOMO of a liquid electrolyte or the valence band of a solid electrolyte to achieve thermodynamic stability against oxidation of the electrolyte by the oxidant. Thermodynamic stability thus introduces the constraint

    μ.sub.A -μ.sub.C ≦E.sub.g                     (4)

as well as the need to match the "window" E_(g) of the electrolyte to the energies μ_(A) and μ_(C) of the reactants to maximize V_(OC). It follows from equations 1 and 2 that realization of a high maximum power P_(max) (equation 5) requires, in addition to as high a V_(OC) as possible, a low internal battery resistance R_(b) (see equation 6).

    P.sub.max =I.sub.max V.sub.max                             (5)

    R.sub.b =R.sub.e1 +R.sub.in (A)+R.sub.in (C)+R.sub.c (A)+R.sub.c (C)(6)

The electrolyte resistance R_(e1) to the ionic current is proportional to the ratio of the effective thickness L to the geometrical area A of the interelectrode space that is filled with an electrolyte of ionic conductivity σ_(i) (equation 7).

    R.sub.e1 =(L/σ.sub.i A)                              (7)

Since ions move diffusively, σ_(i) (see equation 8) increases with temperature. A σ₁ ≦0.1 Scm⁻¹ (the maximum σ_(i) represents the room-temperature protonic conductivity σ_(H) in a strong acid) at an operating temperature T_(op) dictates the use of a membrane separator of large geometrical area A and small thickness L.

    σ.sub.Li =(B/T)exp(-E.sub.α /kT)               (8)

The resistance to transport of the working ion across the electrolyte-electrode interfaces is proportional to the ratio of the geometrical and interfacial areas at each electrode:

    R.sub.in ˜A/A.sub.in                                 (9)

where the chemical reaction of the cell involves ionic transport across an interface, equation 9 dictates construction of a porous, small-particle electrode. Achievement and retention of a high electrode capacity, i.e., utilization of a high fraction of the electrode material in the reversible reaction, requires the achievement and retention of good electronic contact between particles as well as a large particle-electrolyte interface area over many discharge/charge cycles. If the reversible reaction involves a first-order phase change, the particles may fracture or lose contact with one another on cycling to break a continuous electronic pathway to the current collector.

Loss of interparticle electrical contact results in an irreversible loss of capacity. There may also be a reversible capacity fade. Where there is a two-phase process (or even a sharp guest-species gradient at a diffusion front) without fracture of the particles, the area of the interface (or diffusion front) decreases as the second phase penetrates the electrode particle. At a critical interface area, diffusion across the interface may not be fast enough to sustain the current I, so not all of the particle is accessible. The volume of inaccessible electrode increases with I, which leads to a diffusion-limited reversible capacity fade that increases with I. This problem becomes more important at lower ionic conductivity σ_(Li).

The battery voltage V vs. the current I delivered across a load is called the polarization curve. The voltage drop (V_(OC) -V).tbd.η(I) of a typical curve, FIG. 1, is a measure of the battery resistance (see equation 10).

    R.sub.b (I)=η(I)/I                                     (10)

On charging, η(I)=(V_(app) -V_(OC)) is referred to as an overvoltage. The interfacial voltage drops saturate in region (i) of FIG. 1; therefore in region (ii) the slope of the curve is

    dV/dI≈R.sub.e1 +R.sub.c (A)+R.sub.c (C)            (11)

Region (iii) is diffusion-limited; at the higher currents I, normal processes do not bring ions to or remove them from the electrode/electrolyte interfaces rapidly enough to sustain an equilibrium reaction.

The battery voltage V vs. the state of charge, or the time during which a constant current I has been delivered, is called a discharge curve.

Cathode Materials

The cathode material of the present invention, for use in a secondary lithium battery, consists of a host structure into which lithium can be inserted reversibly. The maximum power output, P_(max) (see equation 5), that can be achieved by a cell depends on the open-circuit voltage V_(OC) =ΔE/e and the overvoltage η(I) at the current I_(max) of maximum power

    V.sub.max =V.sub.OC -η(I.sub.max)                      (12)

ΔE is the energy difference between the work function of the anode (or the HOMO of the reductant) and that of the cathode (or the LUMO of the oxidant). In order to obtain a high V_(OC), it is necessary to use a cathode that is an oxide or a halide. It is preferable that the cathode be an oxide in order to achieve a large V_(OC) and good electronic conductivity. To minimize η(I_(max)), the electrodes must be good electronic as well as ionic conductors and they must offer a low resistance to mass transfer across the electrode/electrolyte interface. To obtain a high I_(max), it is necessary to have a large electrode/electrolyte surface area. In addition, where there is a two-phase interface within the electrode particle, the rate of mass transfer across this interface must remain large enough to sustain the current. This constraint tends to limit the electrode capacity more as the current increases.

Oxide host structures with close-packed oxygen arrays may be layered, as in Li_(1-l) CoO₂ (Miztishima, et aL 1980), or strongly bonded in three dimensions (3D) as in the manganese spinels Li_(1-x) Mn₂ !O₄ (Thackeray 1995; Thackeray et al. 1983; Thackeray et al. 1984; Guyomard and Tarascon 1992; and Masquelier et aL 1996). Li intercalation into a van der Waals gap between strongly bonded layers may be fast, but it is can also be accompanied by unwanted species from a liquid electrolyte. On the other hand, strong 3D bonding within a close-packed oxygen array, as occurs in the spinel framework Mn₂ !O₄, offers too small a free volume for the guest Li⁺ ions to have a high mobility at room temperature, which limits I_(max). Although this constraint in volume of the interstitial space makes the spinel structure selective for insertion of Li⁺ ions, it reduces the Li⁺ -ion mobility and hence Li⁺ -ion conductivity σ_(Li). The oxospinels have a sufficiently high σ_(Li), to be used commercially in low-power cells (Thackeray et aL, 1983) but would not be acceptable for the high power cells of the insertion.

The present invention overcomes these drawbacks by providing cathode materials containing larger tetrahedral polyanions which form 3D framework host structures with octahedral-site transition-metal oxidant cations. In the cathode materials of the invention having the NASICON structure, the transition-metal ions are separated by the polyanions, so the electronic conductivity is polaronic rather than metallic. Nevertheless, the gain in σ_(Li) more than offsets the loss in electronic conductivity.

Variation of the energy of a given cation redox couple from one compound to another depends on two factors: (a) the magnitude of the crystalline electric field at the cation, which may be calculated for a purely ionic model by a Madelung summation of the Coulomb fields from the other ions present, and (b) the covalent contribution to the bonding, which may be modulated by the strength of the covalent bonding at a nearest-neighbor counter cation. The stronger is the negative Madelung potential at a cation, the higher is a given redox energy of a cation. Similarly the stronger is the covalent bonding of the electrons at a transition-metal cation, the higher is a given redox energy of that cation. The lower the redox energy of the cation host transition-metal ion, the larger is V_(OC).

The redox couples of interest for a cathode are associated with antibonding states of d-orbital parentage at transition-metal cations M or 4f-orbital parentage at rare-earth cations Ln in an oxide. The stronger is the cation-anion covalent mixing, the higher is the energy of a given LUMO/HOMO redox couple. Modulation of the strength of the cation-anion covalence at a given M or Ln cation by nearest-neighbor cations that compete for the same anion valence electrons is known as the inductive effect. Changes of structure alter primarily the Madelung energy as is illustrated by raising of the redox energy within a spinel M₂ !O₄ framework by about 1 eV on transfer of Li⁺ ions from tetrahedral to octahedral interstitial sites. Changing the counter cation, but not the structure, alters primarily the inductive effect, as is illustrated by a lowering of the Fe³⁺ /Fe²⁺ redox energy by 0.6 eV on changing (MoO₄)²⁻ or (WO₄)²⁻ to (SO₄)²⁻ polyanions in isostructural Fe₂ (XO₄)₃ compounds. Raising the energy of a given redox couple in a cathode lowers the voltage obtained from cells utilizing a common anode. Conversely, raising the redox energy of an anode raises the cell voltage with respect to a common cathode.

The invention provides new cathode materials containing oxide polyanions, including the oxide polyanion (PO₄)³⁻ as at least one constituent, for use in secondary (rechargeable) batteries. For example, the cathode materials of the presented invention may have the general formula LiM(PO₄) with the ordered olivine structure or the more open rhombohedral NASICON framework structure. The cathode materials of the present invention have the general formula LiM(PO₄) for the ordered olivine structure, or Y_(x) M₂ (PO₄)_(y) (XO₄)_(3-y), where 0<y≦3, M is a transition-metal atom, Y is Li or Na and X═Si, As or S and acts as a counter cation, for the rhombohedral NASICON framework structure.

The olivine structure of Mg₂ SiO₄ consists of a slightly distorted array of oxygen atoms with Mg²⁺ ions occupying half the octahedral sites in two different ways. In alternate basal planes, they form zigzag chains of comer-shared octahedra running along the c-axis and in the other basal planes they form linear chains of edge-shared octahedra running also along the c-axis.

In the ordered LiMPO₄ olivine structures of the invention, the M atoms occupy the zigzag chains of octahedra and the Li atoms occupy the linear chains of the alternate planes of octahedral sites. In this embodiment of the present invention, M is preferably Mn, Fe, Co, Ni or combinations thereof. Removal of all of the lithium atoms leaves the layered FePO₄ -type structure, which has the same Pbnm orthorhombic space group. These phases may be prepared from either end, e.g., LiFePO₄ (triphylite) or FePO₄ (heterosite), by reversible extraction or insertion of lithium.

FIG. 2A, FIG. 2B and FIG. 2C show discharge/charge curves vs. lithium at 0.05 mA×cm⁻² (0.95 mA×g⁻¹ and 1.13 mA×g⁻¹, respectively) for Li_(1-x) FePO₄, Li_(1-x) Fe₀.5 Mn₀.5 PO₄ and Li_(x) FePO₄, respectively, where 0≦x≦5. A plateau at 3.4 V corresponds to the Fe³⁺ /Fe²⁺ redox couple and a plateau at 4.1 V corresponds to the Mn³⁺ /Mn²⁺ couple. With LiClO₄ in PC and DME as the electrolyte, it is only possible to charge up a cathode to 4.3 V vs. a lithium anode, so it was not possible to extract lithium from LiMnPO₄, LiCoPO₄ and LiNiPO₄ with this electrolyte. However, in the presence of iron, the Mn³⁺ /Mn²⁺ couple becomes accessible. The inaccessibility is due to the stability of the Mn³⁺ /Mn²⁺, Co³⁺ /Co²⁺ and Ni³⁺ /Ni²⁺ couples in the presence of the polyanion (PO₄)³⁻. The relatively strong covalence of the PO₄ tetrahedron of the compounds of the present invention stabilizes the redox couples at the octahedral sites to give the high V_(OC) 's that are observed.

Insertion of lithium into FePO₄ was reversible over the several cycles studied. FIG. 3 shows discharge/charge curves of FePO₄ /LiClO₄ +PC+DME/Li coin cell at 185 mA·g⁻¹ for FePO₄ prepared by chemical extraction of Li (delithiation) from LiFePO₄. The Li_(x) FePO₄ material of the present invention represents a cathode of good capacity and contains inexpensive, environmentally benign elements. While a nearly close-packed-hexagonal oxide-ion array apparently provides a relatively small free volume for Li⁺ -ion motion, which would seem to support only relatively small current densities at room temperature, increasing the current density does not lower the closed-circuit voltage V. Rather, it decreases, reversibly, the cell capacity. Capacity is easily restored by reducing the current.

As illustrated schematically in FIG. 4, lithium insertion proceeds from the surface of the particle moving inwards behind a two-phase interface. In the system shown, it is a Li_(x) FePO₄ /Li_(1-x) FePO₄ interface. As the lithiation proceeds, the surface area of the interface shrinks. For a constant rate of lithium transport per unit area across the interface, a critical surface area is reached where the rate of total lithium transported across the interface is no longer able to sustain the current. At this point, cell performance becomes diffusion-limited. The higher the current, the greater is the total critical interface area and, hence, the smaller the concentration x of inserted lithium before the cell performance becomes diffusion-limited. On extraction of lithium, the parent phase at the core of the particle grows back towards the particle surface. Thus, the parent phase is retained on repeated cycling and the loss in capacity is reversible on lowering the current density delivered by the cell. Therefore, this loss of capacity does not appear to be due to a breaking of the electrical contact between particles as a result of volume changes, a process that is normally irreversible.

The invention further provides new cathode materials exhibiting a rhombohedral NASICON framework. NASICON, as used herein, is an acronym for the framework host of a sodium superionic conductor Na_(1+3x) Zr₂ (P_(1-x) Si_(x) O₄)₃. The compound Fe₂ (SO₄)₃ has two forms, a rhombohedral NASICON structure and a related monoclinic form (Goodenough et al. 1976; Long et al. 1979). Each structure contains units of two FeO₆ octahedra bridged by three corner-sharing SO₄ tetrahedra. These units form 3D frameworks by the bridging SO₄ tetrahedra of one unit sharing corners with FeO₆ octahedra of neighboring Fe₂ (SO₄)₃ elementary building blocks so that each tetrahedron shares corners with only octahedra and each octahedron with only tetrahedra. In the rhombohedral form, the building blocks are aligned parallel; while they are aligned nearly perpendicular to one another in the monoclinic phase. The collapsed monoclinic form has a smaller free volume for Li⁺ -ion motion which is why the rhombohedral form is preferred. In these structures, the FeO₆ octahedra do not make direct contact, so electron transfer from an Fe²⁺ to an Fe³⁺ ion is polaronic and therefore activated.

Li_(x) Fe₂ (SO₄)₃ has been reported to be a candidate material for the cathode of a Li⁺ -ion rechargeable battery with a V_(OC) =3.6 V vs. a lithium anode (Manthiram and Goodenough 1989). While the sulfates would seem to provide the desired larger free volume for Li, batteries using sulfates in the cathode material tend to exhibit phase-transition problems, lowering the electronic conductivity. The reversible lithium insertion into both rhombohedral and monoclinic Fe₂ (SO₄)₃ gives a flat closed-circuit voltage vs. a lithium anode of 3.6 V (Manthiram and Goodenough 1989; Okada et al. 1994; Nanjundaswamy et al. 1996). Neither parent phase has any significant solid solution with the orthorhombic lithiated phase Li₂ Fe₂ (SO₄)₃, which is derived from the rhombohedral form of Fe₂ (SO₄)₃ by a displacive transition that leaves the framework intact. Powder X-ray diffraction verifies that lithiation occurs via a two-phase process (Nanjundaswamy et al. 1996). Increasing the current density does not change significantly the closed-circuit voltage V, but it does reduce reversibly the capacity. The reduction in capacity for a given current density is greater for the motion of the lithiated interface. The interstitial space of the framework allows fast Li⁺ -ion motion, but the movement of lithium across the orthorhombic/monoclinic interface is slower than that across the orthorhombic/rhombohedral interface, which makes the reversible loss of capacity with increasing current density greater for the monoclinic than for the rhombohedral parent phase.

The cathode materials of the invention avoid the phase transition of known sulfate cathode materials by incorporating one or more phosphate ions as at least one of the constituents of the cathode material. The rhombohedral R3c (NASICON) and monoclinic P2₁ /n framework structures of Li₃ Fe₂ (PO₄)₃ are similar to those for the sulfates described above, as illustrated in FIG. 5A and FIG. 5B.

A further embodiment of the invention is a rhombohedral NASICON cathode material having the formula A_(3-x) V₂ (PO₄)₃, where A may be Li, Na or a combination thereof. Rhombohedral A_(3-x) V₂ (PO₄)₃ reversibly intercalates 1.5 Li per formula unit for a discharge capacity of 100 mAh·g⁻¹ with average closed-circuit voltage being 3.8 V vs. a lithium anode (see FIG. 8). The voltage and capacity performances of the rhombohedral A₃₋₂ V₂ (PO₄)₃ compounds of the invention are comparable to the high-voltage cathode materials LiMn₂ O₄ (4.0 V), LiCoO₂ (4.0 V) and LiNiO₂ (4.0 V). The large, open three-dimensional framework of VO₆ octahedra and PO₄ tetrahedra allows an easy diffusion of the lithium ions, making it attractive for high-power batteries. A further advantage of this material is that it includes a cheaper and less toxic transition-metal element (V) than the already developed systems using Co, Ni, or Mn.

EXAMPLES Example 1 Ordered Olivine LiMPO₄ Compounds

The ordered-olivine compound LiFePO₄ was prepared from intimate mixtures of stoichiometric proportions of Li₂ CO₃ or LiOH·H₂ O, Fe{CH₂ COOH}₂ and NH₄ H₂ PO₄ ·H₂ O; the mixtures were calcined at 300-350° C. to eliminate NH₃, H₂ O, and CO₂ and then heated in Ar at about 800° C. for 24 hours to obtain LiFe PO₄. Similar solid-state reactions were used to prepare LiMnPO₄, LiFe_(1-x) Mn_(x) PO₄, LiCoPO₄ and LiNiPO₄. FePO₄ was obtained from LiFePO₄ by chemical extraction of Li from LiFePO₄. Charge/discharge curves for Li_(1-x) FePO₄ and discharge/charge cycles for Li_(x) FePO₄ gave similar results with a voltage of almost 3.5 V vs. lithium for a capacity of 0.6 Li/formula unit at a current density of 0.05 mA·cm²⁻ (See FIG. 2A and FIG. 2C). The electrolyte used had a window restricting voltages to V<4.3 V. Li extraction was not possible from LiMnPO₄, LiCoPO₄, and LiNiPO₄ with the electrolyte used because these require a voltage V>4.3 V to initiate extraction. However, Li extraction from LiFe_(1-x) Mn_(x) PO₄ was performed with 0≦x≦0.5, and the Mn³⁺ /Mn²⁺ couple give a voltage plateau at 4.0 V vs. lithium.

Example 2 Rhombohedral NASICON Li_(x) M₂ (PO₄)₃ Structures

The inventors compared redox energies in isostructural sulfates with phosphates to obtain the magnitude of the change due to the different inductive effects of sulfur and phosphorus. Rhombohedral Li_(1+x) Ti₂ (PO₄)₃ has been shown to exhibit a flat open-circuit voltage V_(OC) =2.5 V vs. lithium, which is roughly 0.8 V below the Ti⁴⁺ /Ti³⁺ level found for FeTi(SO₄)₃. The flat voltage V(x) is indicative of a two-phase process. A coexistence of rhombohedral and orthorhombic phases was found for x=0.5 (Delmas and Nadiri 1988; Wang and Hwu 1992). Li_(2+x) FeTi(PO₄)₃ of the present invention remains single phase on discharge.

All three phosphates Li₃ M₂ (PO₄)₃, where M=Fe, Fe/V, or V, have the monoclinic Fe₂ (SO₄)₃ structure if prepared by solid-state reaction. The inventors have found that these compounds exhibit a rhombohedral structure when prepared by ion exchange in LiNO₃ at 300° C. from the sodium analog Na₃ Fe₂ (PO₄)₃. The discharge/charge curve of FIG. 6A for lithium insertion into rhombohedral Li_(3+x) Fe₂ (PO₄)₃ exhibits an average V_(OC) of 2.8 V. This is surprisingly different from the curves for the monoclinic form (See FIG. 6B). The inventors have found that up to two lithiums per formula unit can be inserted into Li₃ Fe₂ (PO₄)₃, leading to Li₅ Fe₂ (PO₄)₃. The Li⁺ -ion distribution in the interstitial space of Li_(3+x) Fe₂ (PO₄)₃, where 0<x<2, appears to vary continuously with x with a high degree of disorder. FIG. 7A shows a reversible capacity loss on increasing the current density from 0.05 to 0.5 mA·cm⁻². A reversible discharge capacity of 95 mAh·g⁻¹ is still observed for rhombohedral Li_(3+x) Fe₂ (PO₄)₃ at a current density of 20 mA·g⁻¹. This is much reduced compared to what is encountered with the monoclinic system (See FIG. 7B). With a current density of 23 mA·g⁻¹ (or 1 mA·cm⁻²), the initial capacity of 95 mAh·g⁻¹ was maintained in a coin cell up to the 40th cycle.

Another cathode material of the present invention, Li₂ FeTi(PO₄)₃, having the NASICON framework was prepared by solid-state reaction. This material has a voltage ranging from 3.0 to 2.5 V.

Rhombohedral TiNb(PO₄)₃ can be prepared by solid-state reaction at about 1200° C. Up to three Li atoms per formula unit can be inserted, which allows access to the Nb⁴⁺ /Nb³⁺ couple at 1.8 V vs. lithium for x>2 in Li_(x) TiNb(PO₄)₃ Two steps are perhaps discernible in the compositional range 0<x<2; one in the range of 0<x<1 corresponds to the Ti⁴⁺ /Ti³⁺ couple in the voltage range 2.5 V<V<2.7 V and the other for 1<X<2 to the Nb⁵⁺ /Nb⁴⁺ couple in the range 2.2 V<V<2.5 V. It appears that these redox energies overlap. This assignment is based on the fact that the Ti⁴⁺ /Ti³⁺ couple in Li_(1tx) Ti₂ (PO₄)₃ gives a flat plateau at 2.5 V due to the presence of two phases, rhombohedral LiTi₂ (PO₄)₃ and orthorhombic Li₃ Ti₂ (PO₄)₃. The presence of Nb in the structure suppresses the formation of the second phase in the range 0<x<2.

Rhombohedral LiFeNb(PO₄)₃ and Li₂ FeTi(PO₄)₃ can be prepared by ion exchange with molten LiNO₃ at about 300° C. from NaFeNb(PO₄)₃ and Na₂ FeTi(PO₄)₃, respectively. Two Li atoms per formula unit can be inserted reversibly into Li_(2+x) FeTi(PO₄)₃ with a little loss of capacity at 0.5 mA·cm⁻². Insertion of the first Li atom in the range 2.7 V<V<3.0 V corresponds to the Fe³⁺ /Fe²⁺ redox couple and of the second Li atom in the range of 2.5 V<V<2.7 V to an overlapping Ti⁴⁺ /Ti³⁺ redox couple. The insertion of lithium into Li_(1+x) Fenb (PO₄)₃ gives a V vs. x curve that further verifies the location of the relative positions of the Fe³⁺ /Fe²⁺, Nb⁵⁺ /Nb⁴⁺ redox energies in phosphates with NASICON-related structures. It is possible to insert three lithium atoms into the structure; and there are three distinct plateaus corresponding to Fe³⁺ /Fe²⁺ at 2.8 V, Nb⁵⁺ /Nb⁴⁺ at 2.2 V, and Nb⁴⁺ /Nb⁵⁺ at 1.7 V vs. lithium in the discharge curve.

The rhombohedral A_(3-x) V₂ (PO₄)₃ compounds of the invention can be prepared by ionic exchange from the monoclinic sodium analog Na₃ V₂ (PO₄)₃. The inventors were also able to prepare the rhombohedral Li₂ NaV₂ (PO₄)₃ with the NASICON framework by a direct solid-state reaction (FIG. 9). The discharge/charge curves at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the rhomobhedral Li_(x) NaV₂ (PO₄)₃ are shown in FIG. 8.

The rhombohedral LiFe₂ (SO₄)₂ (PO₄) may be prepared by obtaining an aqueous solution comprising FeCl₃, (NH₄)₂ SO₄, and LiH₂ PO₄, stirring the solution and evaporating it to dryness, and heating the resulting dry material to about 500° C. Discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for rhombohedral Li_(1+x) Fe₂ (PO₄)(SO₄)₂, where 0<x<3, are shown in FIG. 10.

All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and structurally related may be substituted for the agents described herein to achieve similar results. All such substitutions and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.

References

The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.

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What is claimed is:
 1. A cathode material for a rechargeable electrochemical cell, said cell also comprising an anode and an electrolyte, the cathode comprising a compound having the formula LiMPO₄, where M is at least one first-row transition-metal cation.
 2. The cathode material of claim 1, where M is further defined as being selected from the group consisting of Mn, Fe, Co, and Ni.
 3. The cathode material of claim 1, where M is further defined as being a combination of cations, at least one of which is selected from the group consisting of Mn, Fe, Co and Ni.
 4. The cathode material of claim 3, where M is Fe_(1-x) Mn_(x) or Fe_(1-x) Ti_(x) and 0<x<1.
 5. The cathode material of claim 2, wherein the cathode material has the formula LiFePO₄.
 6. A secondary battery comprising an anode, a cathode and an electrolyte, said cathode comprising an ordered olivine compound having the formula LiMPO₄, where M is at least one first-row transition-metal cation.
 7. The battery of claim 6, where M is further defined as being selected from the group consisting of Mn, Fe, Co, and Ni.
 8. The battery of claim 6, where M is further defined as being a combination of cations, at least one of said cations being selected from the group consisting of Mn, Fe, Co, and Ni.
 9. The battery of claim 8, wherein M is Fe_(1-x) Mn_(x) or Fe_(1-x) Ti_(x), where 0≦x≦1. 